Update usage docs to v2.1.1

Author: dwijnand

README.md

@@ -8,7 +8,7 @@ This library provides some of the new APIs from Scala 2.13 to Scala 2.11 and 2.1
 To use this library, add the following to your build.sbt:
 
 ```
-libraryDependencies += "org.scala-lang.modules" %% "scala-collection-compat" % "2.0.0"
+libraryDependencies += "org.scala-lang.modules" %% "scala-collection-compat" % "2.1.1"
 ```
 
 Version 2.0.0 is compatible with Scala 2.13, 2.12, and 2.11.